It is sometimes necessary during medical procedures to insert a device into blood vessels, heart valves, urinary tracts or other conduits of the human body in order to either hold open or widen those conduits. These devices are generally referred to as "stents." A number of the stents known in the art take the form of a helical coil and are designed for permanent implantation. An example of such a permanently implantable helical coil stent is shown in U.S. Pat. No. 5,201,901, issued to Harada. In the Harada patent, a stent is constructed in one embodiment out of a unidirectional memory alloy, such as heat activated nickel-titanium alloy ("heat/cool Nitinol"), which is responsive to body heat. After such a stent is inserted into the appropriate location in the body with the aid of a guide wire or catheter, the body heat causes it to expand to a predetermined or "memorized" size. The guide wire or catheter is then withdrawn to allow the stent to remain permanently implanted.
In certain medical procedures where stents are needed, it would be impractical to leave the stent permanently implanted. One of these medical procedures is the heart bypass procedure with heart assist described in U.S. Pat. No. 4,889,137, issued to co-inventor Kolobow, the disclosures of which are incorporated by reference. In such a heart bypass procedure, a stent is used to temporarily keep one or both of the pulmonary and tricuspid valves partially or totally open. This stent is needed to provide effective decompression of the left heart during the heart bypass procedure in order to prevent the onset of potentially lethal forms of acute pulmonary edema. Of course, if the heart is to return to its normal function after the heart bypass procedure, it is necessary for the stent to be adjusted back to a contracted state and completely removed so that the heart valves can return to their normal function.
Several helical coil stents in the art have already been made to be temporary and adjustable. These include the temporary, adjustable helical coil stents of U.S. Pat. No. 4,553,545, issued to Maass, and of U.S. Pat. No. 5,342,300, issued to Stefanadis. In both the Stefanadis and Maass patents, adjustable helical coil stents are made of bare stainless steel. In his patent, Maass recognizes that thin stainless steel coils have a tendency in an expanded state to be mechanically unstable, even at the modest expansion ratios of 0.5 to 3.8 described in his specification. This instability results in the tilting or collapse of the coil after its operational expansion. Maass tries to address this problem by adding a cumbersome set of additional parts which he refers to as "rigidifying means."
What is needed in the art is a simple, highly elastic, adjustable helical coil stent which does not become mechanically unstable when placed in an expanded state. This is particularly needed for heart bypass procedures where the helical coil stent must be greatly compressed to be inserted into a narrow blood vessel and then reliably expanded to hold open a heart valve. The expansion ratios involved in such a heart bypass procedure can be significantly greater that the 0.5 to 3.8 range described by Maass.
What is also needed in the art is a simple, highly elastic, adjustable helical coil stent which will not promote blood clotting or blood vessel damage through the contact of sharp metallic surfaces with soft blood vessel tissue.